The present invention relates to optical pickup devices and, more specifically, to an optical pickup device performing recording and/or reproducing for a digital video disk (a DVD) and a compact disk (a CD).
An optical disk with a thickness of about 1.2 mm, such as a CD-ROM (Compact Disk-Read Only Memory), has been provided for reading information using a semiconductor laser. In this type of optical disk, by performing focusing servo control and tracking servo control for an objective lens for pickup, a pit string of a signal recording surface is irradiated with laser beam to reproduce a signal. Recently, such optical disks are increasingly becoming higher in density for recording an animation lasting for a long period of time.
For example, a DVD standard has been proposed for recording information of 4.7 G bytes on one side of the optical disk having a diameter of 12 cm, which is the same as that of the CD-ROM. A transparent substrate of the DVD has a thickness of about 0.6 mm. A single DVD, which has two such transparent substrates applied to each other back to back, can record information of 9.4 G bytes. Further, as a write-once optical disk having the same diameter, thickness of substrate and recording density as those for the CD-ROM, there exists a CD-R (Compact Disk-Recordable).
Since three different types of optical disks, including the DVD, the CD-ROM and the CD-R, would exist in the same apparatus in the near future, a device capable of compatibly reproducing these three different types of optical disks is required. The DVD and the CD-ROM or the CD-R, however, cannot be reproduced by a single optical pickup device because of the difference in thicknesses of the transparent substrates.
Then, in Japanese Patent Laying-Open No. 5-303766, an apparatus has been proposed which allows reproduction of a high density optical disk having a thin transparent substrate with a thickness of 0.6 mm and a standard density optical disk having a standard transparent substrate with a thickness of 1.2 mm by using a single optical pickup device. In the apparatus, an objective lens with a numerical aperture of 0.6 is employed which has been designed to reproduce the high density optical disk by a laser beam with a short wavelength. When the standard density optical disk is to be reproduced, an aspherical optical device is inserted on the side of a light source of the objective lens which is provided with an aperture for shielding the periphery of laser beam to decrease an effective numerical aperture of the objective lens.
Moreover, to change the effective numerical aperture of an objective lens selectively shielding the periphery of laser beam emitted from a semiconductor laser to collect laser beam, an apparatus has been disclosed in Japanese Patent Laying-Open No. 8-321065 which is provided with a liquid crystal selectively rotating the plane of polarization of laser beam and a polarizing plate allowing transmission of only laser beam which is polarized in a specific direction, and which can compatibly reproduce optical disks having substrates with different thicknesses. While the apparatus can compatibly reproduce the DVD and the CD-ROM having substrates of different thicknesses, it cannot reproduce the CD-R as a laser beam with a wavelength of 635 nm is employed. The reason is as follows.
FIG. 1 is a diagram showing a relation between a pit depth and intensity of reflected light for every laser beam with a different wavelength. As shown in FIG. 1, when the laser beam with a wavelength of 635 nm is employed, the intensity of reflected light is the highest with the pit depth of about 105 nm. On the other hand, when laser beam with a wavelength of 780 nm is employed, the intensity of reflected light is the highest with the pit depth of about 125 nm. In the case of the CD-R, reflectance significantly changes with the wavelength of laser beam as organic dye is used for a recording film and a sufficient intensity of reflected light cannot be obtained using a single-wavelength laser with a wavelength of 635 nm. Thus, the CD-R cannot be suitably reproduced. Therefore, a two wavelengths to beam laser is required for the optical pickup device capable of compatibly reproducing the DVD and the CD-R or the CD-ROM. When laser beam with a wavelength of 430 nm would be employed due to the trend of shorter wavelength in the near future, such a two wavelengths to beam laser would be even more highly required.
Therefore, it is an object of the present invention to provide an optical pickup device capable of performing recording and/or reproducing for optical disks having substrates with different thicknesses by using a laser beam with two different wavelengths.
According to the present invention, an optical pickup device performing recording and/or reproducing for a first optical disk having a first transparent substrate and a second optical disk having a second transparent substrate with a thickness which is smaller than that of the first transparent substrate includes an objective lens, a laser beam generating means and an optical device. The objective lens is arranged opposite to the first or second optical disk. The laser beam generating means selectively generates a first laser beam with a first wavelength and a second laser beam with a second wavelength which is different from the first wavelength. The optical device includes: a central region arranged between the objective lens and the laser beam generating means, allowing transmission of the first laser beam without any change and increasing the diameter of the second laser beam by diffraction; and a peripheral region allowing transmission of the first laser beam without any change and substantially shielding the second laser beam by diffraction or absorption.
Preferably, a hologram is formed in the central region of the optical device.
More preferably, the hologram includes a plurality of annular convex portions each having four steps and coaxially formed, where a height h1 of each step is determined in accordance with the following expressions (1) to (5).                               η          m                =                              "LeftBracketingBar"                                          1                T                            ⁢                                                ∫                  0                  T                                ⁢                                                      A                    ⁡                                          (                      x                      )                                                        ⁢                                      xe2x80x83                                    ⁢                  exp                  ⁢                                      {                                          i                      ⁢        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Here, xcex7m is mth order diffraction efficiency, xcfx86(x) is a function of phase difference defined by expression (2), T is a period of the function of phase difference, A(x) is transmittance, xcex is the first, or second wavelength, n is refractive index of the annular convex portion, n0 is refractive index of the periphery of the annular convex portion and xcfx860 is a constant.
Preferably, a refraction grating is formed in the peripheral region of the optical device.
More preferably, the diffraction grating has varying grating constants.
Preferably, the laser beam generating means polarizes the first laser beam in a first direction and the second laser beam in a second direction which is different from the first direction. In the peripheral region of the optical device, a polarizing filter is formed having a polarizing direction which is perpendicular to the second direction.
Preferably, a polarizing glass is formed in the peripheral region of the optical device which absorbs the laser beam with the second wavelength.
Preferably, the laser beam generating means is a semiconductor laser including a package and first and second laser chips. The first laser chip is arranged within the package and oscillates the first laser beam. The second laser beam is arranged within the package and oscillates the second laser beam.
More preferably, the optical pickup device moves along two parallel guide shafts provided in a radial direction of the first or second optical disk, and includes a rising mirror and a body. The rising mirror is arranged immediately below the objective lens and the optical device, and reflects the first or second laser beam directed from the semiconductor laser in a direction which is parallel to the main surface of the first or second optical disk in a direction which is perpendicular to the main surface of the first or second optical disk. The body is supported between the two guide shafts and the objective lens accommodates the optical device, the semiconductor laser and the rising mirror. An optical axis of the laser beam entering the rising mirror is angled with respect to a perpendicular relative to the two guide shafts to form an acute angle.
More preferably, a line passing emittance openings of the first and second laser chips is angled with respect to the main surface of the first or second optical disk to form an acute angle which is equal to the above mentioned acute angle.
Preferably, the optical pickup device further includes a collimator lens arranged between the optical device and the semiconductor laser. The first laser chip is spaced by a first distance from the collimator lens such that the first laser beam transmitted through the collimator lens is collimated. The second laser chip is spaced by a second distance, which is different from the first distance, from the collimator lens such that the second laser beam transmitted through the collimator lens is collimated.
Preferably, the semiconductor laser further includes an optical waveguide. The optical waveguide includes a first incident opening facing the emittance opening of the first laser chip, a second incident opening facing the emittance opening of the second laser chip and an emittance opening communicating with the first and second incident openings.
Preferably, the first and second laser chips are arranged such that one sides thereof are adjacent to each other. The distances between the emittance openings of the first and second laser chips and the one sides are respectively shorter than the distances between the emittance openings and the other sides opposite to the one sides.
Preferably, the semiconductor laser further includes a photodetector arranged on the side opposite to the side of emittance of the first and second laser chips for monitoring both the first and second laser beams leaked from the first and second laser chips.
Preferably, the semiconductor laser further includes first to fourth terminals. The first terminal is connected to one electrodes of the first and second laser chips and the photodetector. The second terminal is connected to the other terminal of the second laser chip. The third terminal is connected to the other electrode of the second laser chip. The fourth terminal is connected to the other electrode of the photodetector.
Preferably, the first wavelength is between 620 nm and 680 nm and the second wavelength is between 765 nm and 795 nm.
Preferably, the objective lens is adapted to the first optical disk and has a numerical aperture of between 0.55 and 0.65.
Preferably, the objective lens has an effective numerical aperture of between 0.40 and 0.50 upon incidence of the second laser beam.
Thus, in recording and/or reproducing of the first optical disk, the first laser beam is transmitted through the optical device without any change and focused on a signal recording surface of the first optical disk by the objective lens. On the other hand, in recording and/or reproducing of the second optical disk, the periphery of the second laser beam is substantially shielded by the peripheral region of the optical device and the diameter of the central portion of the second laser beam is increased by the central region of the optical device, so that the second laser beam is focused on the signal recording surface of the second optical disk by the objective lens. Thus, the optical pickup device is capable of performing recording and/or reproducing for the first and second optical disks having substrates with different thicknesses.